The present invention relates to an anti creep device for an automatic transmission for a vehicle, and more particularly relates to such an anti creep device which restores anti creep action more slowly than it applies, it, and also to such an anti creep device which provides a pattern of anti creep action of a particular beneficial sort.
A vehicle equipped with an automatic transmission has a tendency, when stationary with the engine running in the idling condition and with its manual range selection means such as a range shifting lever set to a forward drive position such as drive range, to move forward against the intention of the vehicle operator, due to the dragging torque of the fluid coupling such as a torgue converter; this is called creep. This creep phenomenon is in general undesirable, since it is outside the control of the vehicle operator and typically requires him or her to keep the brake pedal of the vehicle depressed in order to prevent vehicle motion. Also, when the vehicle is not moving, such creep applies a braking force on the engine of the vehicle, and thus necessitates so called idle up to be performed, in which the throttle valve of the engine is somewhat opened as compared to its setting when the transmission is in neutral range, in order to compensate for this braking action. This deteriorates the idling fuel economy of the vehicle, and also complicates the control structures therefor.
Accordingly, it has been recognized that, when the vehicle is stationary with the engine running in the idling condition and the transmission set to the drive range, it is desirable to cut off the power transfer between the engine and the drive wheels of the vehicle completely by setting the transmission to a neutral state, so as to be able to permanently keep the throttle valve set to a low setting, and so as to prevent any undesired forward motion of the vehicle; and in the past various anti creep devices have been proposed to this end. A typical such prior art device is one in which the stationary state of the vehicle is detected and the actuating pressure of a friction engagement device (such as a clutch) which provides the first speed stage is kept below its engagement pressure, so as to not engage the first speed stage at all in these conditions. Then, when the vehicle is to be moved away from rest, the operator presses down on the accelerator pedal thereof, and it is arranged that the actuating pressure of the friction engagement device is controlled in proportion to the amount of depression of said accelerator pedal.
However, providing an actuating pressure for the friction engagement device which is simply proportional to the amount of depression of the accelerator pedal is liable to cause some inconvenience in some circumstances. For example, if a vehicle with an automatic transmission is being driven in congested traffic on an incline such as a hill, then sometimes a driver will operate the vehicle by stopping it and starting it and driving it at low speed, only by using movement of the throttle pedal, so as to keep an appropriate distance from the car in front; and in such an operational mode, if the actuating pressure for said friction engagement device is simply proportional to accelerator pedal depression, there is a risk that said friction engagement device will be kept half engaged and slipping for a considerable time, which can severely deteriorate its durability and that of the transmission as a whole. A similar thing happens if the vehicle is started off from rest by using the so called jackrabbit starting technique, in which the accelerator pedal and the brake pedal are both fully depressed and then the brake pedal is suddenly released. These situations can be avoided by the driver by proper and appropriate vehicle operation, but this would mean that the driver would be required to modify his or her driving habits, which is not reasonable and not realistic in terms of vehicle design.
Another consideration that has come to be realized is that the timing for transiting between the creeping state of the vehicle and the non creeping state in which anti creep action is being provided should not be symmetrical. In fact, when the anti creep action is being provided, and then the accelerator pedal of the vehicle is relatively quickly depressed, it is necessary to stop the anti creep action as quickly as possible and to restore the creeping state of the vehicle, in preparation for a possibly fairly abrupt start off, in order not to impair transmission responsiveness. On the other hand, when creeping action is being provided for the vehicle and then it becomes desirable to restore the anti creeping state, if this restoration is performed too quickly some unpleasant shock tends to be generated, and it is desirable to provide a moderate time delay. An electrical system for providing such a time delay or time lag when restoring anti creep action on detecting the stationary state of the vehicle could be conceived of, but this system would require a solenoid valve, and so an alternative system would be desirable. Further, in any case the analog type control fuction of a hydraulic fluid pressure system is generally understood to be better than the digital or on/off control function of an electrical system, in view of the desirability of improving the feeling of starting off of the vehicle.